To choose an air-to-water heat pump well, start from the building, not the appliance: <strong>calculated heat losses, the lowest possible flow temperature, suitable emitters and a well-placed outdoor unit</strong>. Brand and model come afterwards. This guide gives you the reference points to compare quotes and ask the right questions.
The quick answer: five points to validate
An air-to-water heat pump draws heat from outdoor air to warm the water in your heating circuit and, often, your domestic hot water. Five points determine the result:
- <strong>Output</strong>: derived from a heat-loss calculation, not a per-square-metre rule.
- <strong>Flow temperature</strong>: the lower it is, the better the efficiency.
- <strong>Emitters</strong>: underfloor heating, low-temperature radiators or existing radiators, to be checked room by room.
- <strong>SCOP</strong>: compare it under the same temperature condition.
- <strong>Noise and placement</strong>: sound power in dB(A) and distance to neighbours and bedrooms.
Air-to-air, air-to-water, ground-to-water, monobloc or split
An air-to-air heat pump blows warm or cool air through indoor units. It does not heat the water of a radiator circuit. An air-to-water unit feeds a hydraulic circuit (radiators, underfloor heating, cylinder): the usual solution to replace a boiler. A ground-to-water unit draws heat from the ground through probes or collectors; it needs drilling or excavation and suitable land.
An air-to-water unit comes in two architectures. A monobloc houses the whole refrigerant circuit in the outdoor unit: only water enters the house, but outdoor pipes must be protected from frost. A split links an outdoor unit to an indoor module through refrigerant pipes: water stays indoors, but the refrigerant circuit must be installed by a qualified technician.
COP and SCOP: what these figures measure
The COP is the ratio of heat delivered to electricity consumed at one moment, under precise test conditions (outdoor air and water temperatures). A COP of 4 gives 4 kWh of heat for 1 kWh of electricity.
The SCOP is the seasonal version: it estimates efficiency over a whole heating season for a European reference climate. The EU ecodesign (EU 813/2013) and energy labelling (EU 811/2013) regulations express this seasonal efficiency (ηs) and require the product's sound power level to be stated.
These regulations distinguish two conditions: the low-temperature application (water at 35 °C at the indoor heat exchanger outlet) and the medium-temperature application. A SCOP only makes sense with its condition: comparing a SCOP at 35 °C with one at a higher temperature skews the choice. The reference climate is not exactly yours: real efficiency also depends on the installation and its settings.
Flow temperature and emitters: the deciding factor
The hotter the water for the emitters must be, the more electricity the heat pump uses for the same heat. The aim is therefore to heat with the coolest water possible.
Underfloor heating works at low temperature: the most favourable pairing. Low-temperature radiators and fan coil units have a large exchange surface and accept cooler water. Existing radiators, sized for a high-temperature boiler, may require water too hot for good efficiency.
In renovation, there are two levers: improve insulation first (the need falls, and so does the required temperature) and replace only the radiators in rooms where the emitted output is insufficient. Have the flow temperature really required in severe cold calculated room by room.
Sizing the output with a heat-loss calculation
A « so many watts per m² » rule only gives an order of magnitude. The serious starting point is the building's heat-loss calculation (method of standard EN 12831), at the chosen design outdoor temperature, taking account of insulation, glazing, ventilation and heated volume.
An undersized unit will not cover cold days and falls back on its electric backup. An oversized one starts and stops too often, which wears the compressor and lowers efficiency. Variable-speed (inverter) compressors modulate their output, but do not justify oversizing.
If you are about to insulate, size for the situation after the works. Ask to see the calculation, not just a quoted output. For domestic hot water, the heat pump is paired with a cylinder: its volume depends on the number of occupants and their habits, and its insulation matters.
Noise: reading dB(A) and placing the outdoor unit well
The energy label states the sound power level (LWA) in dB(A), as defined by the EU regulations. It is a characteristic of the appliance, not the noise heard at home: the sound pressure you perceive depends on distance, obstacles and reflective surfaces such as nearby walls or corners.
Place the unit away from bedroom windows and property boundaries, avoid corners between two walls, provide a stable base and a condensate drain. Ask your municipality or region about any noise or distance rules for outdoor installations. Compare the sound power of equivalent models, and see whether a night mode exists.
Common mistakes
- Choosing output per square metre, without a heat-loss calculation.
- Comparing SCOP values established at different flow temperatures.
- Keeping old radiators without checking the temperature needed in severe cold.
- Confusing the label's sound power with the noise neighbours hear.
- Placing the outdoor unit in a corner, against a bedroom or near the property boundary.
- Sizing for today's state when insulation is planned.
- Signing a quote without checking your region's incentive conditions.
What to check before buying
On the datasheet and label, look for: the type (air-to-water, monobloc or split), the rated output and its temperature condition, the seasonal efficiency and its condition (low or medium temperature), the sound power level in dB(A) and whether there is an electric backup heater.
For the installation, ask for the heat-loss calculation, the planned flow temperature, the outdoor unit location, the cylinder type and the commissioning settings. Heat pumps contain a refrigerant covered by the EU regulation on fluorinated gases (EU 2024/573): work on the refrigerant circuit requires certification of the person or company. Ask for the certificate.
Incentives are regional: in Wallonia, contact SPW Énergie; in Flanders, the Flemish Energy and Climate Agency (VEKA); in Brussels, Brussels Environment. Conditions and procedures change, and some steps come before the works.
Frequently asked questions
Does an air-to-water heat pump work with existing radiators?
Sometimes. It depends on the flow temperature needed in severe cold. A room-by-room calculation shows whether the radiators suffice, whether some must be replaced or whether better insulation is needed.
What is the difference between COP and SCOP?
COP is measured at one moment under precise conditions. SCOP estimates efficiency over a full heating season for a reference climate.
Why does flow temperature matter so much?
The hotter the water must be, the more electricity the heat pump uses for the same heat. Emitters that run at low temperature give better efficiency.
What do the dB(A) on the label mean?
They give the appliance's sound power level. The noise you hear at home depends on distance, placement and surroundings.
Who can install a heat pump?
A competent installer. For any work on the refrigerant circuit, the person or company must hold the certification required by EU rules on fluorinated gases. Also check your region's incentive conditions.
